Corrosion resistant core-plate and coatings therefor

ABSTRACT

An improved coating for electrical or magnetic steel core-plates, having corrosion resistance comprises adding to a known combination of organic quaternary ammonium silicate and ethylene/acrylic or ethylene/vinyl acetate copolymer a small amount of a barium, strontium or lead chromate.

BACKGROUND OF THE INVENTION

This invention is an improvement on the inventions disclosed and claimedin Parkinson U.S. Pat. Nos. 3,839,256 and 3,908,066 entitled"Silicate-Resin Coating Composition" and "Protecting Metal and MetalProducts", respectively. The Parkinson patents describe compositions,methods and coated electrical or magnetic grade steel for use inmagnetic cores of transformers, motors and the like. The coatingsdescribed in the Parkinson patents have been used widely andsuccessfully in commerce. However, there is still a problem evident fromtime to time with corrosion of the coated steel plate, particularlyduring storage and/or in use involving continuous viscissitudes oftemperature and humidity. While a small amount of corrosion does notsignificantly affect the performance of the product in motors,generators and transformers, it hampers the salability of the productand generally raises questions of confidence by purchasers and users.When the surface is corroded, also, there is a tendency for thesilicate-resin composition to separate from the plate.

The Parkinson patents disclose and claim the use of a class ofquaternary ammonium silicates in combination with certain ethylenecopolymers, terpolymers and comonomers, applied in an aqueousdispersion, to provide heat-resistant electrical insulation andlubricity. The complete specifications of U.S. Pat. Nos. 3,839,256 and3,908,066 are incorporated herein by reference. The products theydisclose have been successful in all respects including adhesion,toughness, lack of abrasiveness and relative thinness of coating;however, corrosion resistance has been difficult to impart to thesubstrate at the same time, along with the other desirable attributes.

SUMMARY OF THE INVENTION

My invention comprises the use of a chromate selected from the groupconsisting of strontium chromate, barium chromate and lead chromate inamounts from as little as 0.1% to 5.0% or more, together with thecoating compositions described in the Parkinson patents, applied to themetal substrate in the same manner. Preferably, my new compositionincludes from about 2% to about 5% by weight of strontium, barium orlead chromate based on the two components (hereinafter sometimesreferred to as a "Parkinson formulation") of claim 1 of U.S. Pat. No.3,839,256, as follows:

A solution or dispersion in an aqueous medium of

a. quaternary ammonium silicate of the formula WM₂ O.X(N_(n) R_(p))₂ O.YSiO₂ Z H₂ O where W is a number from 0 to 1, X is a number from 0.33 to1.5, Y is a number from 2 to 10, Z is a number from 1 to 20, M is analkali metal, R is an organic radical that forms an NR base selectedfrom alkyl, alkanol, and heterocyclic groups, each independentlyselected, having from 1 to 20 carbon atoms, n is an integer from 1 to10, and p is at least 4, and

b. ethylene polymer selected from the group consisting of copolymers,terpolymers and ionomers of ethylene with acrylic, methacrylic orcrotonic acids or esters thereof or vinyl acetate, wherein the portionof ethylene polymer derived from ethylene is from about 60 to about 90weight percent, the weight ratio of SiO₂ derived from the quaternaryammonium silicate to ethylene polymer being from about 20:1 to about1:20, and

c. about 0.1% to about 5%, based on the total of a and b, of a chromateselected from the group consisting of barium chromate, strontiumchromate, and lead chromate.

A preferred formulation employs methyl triethanol ammonium silicatehaving a molar ratio of about 12.5SiO₂ to the organic cation and anon-volatile silica weight of about 45 percent.

Preferred ethylene polymers contain about 70-85% ethylene and are usedin solutions or dispersions of about 15-30% solids in ammonia.

A preferred ratio of ethylene polymer to silica solids ranges from about0.3 to about 0.7.

Previous research records indicated that chromate salts could not beused to impart corrosion inhibition to coreplate coatings because theycaused the coreplate solutions or dispersions to quickly form rubbery,irreversible gels. These indications were investigated as follows:

100 grams of a Parkinson formulation containing the specific ingredientslisted in Table I was mixed with 5 grams of each of the various chromatesalts listed in Table II wetted with 1.5 gram of water to aid inthoroughly dispersing the chromate in the silicate-resin medium. Themixtures were set aside to observe the consequences of the combinationof coating and chromate. The observations are also recorded in Table II.

                  TABLE I                                                         ______________________________________                                                                Parts by                                                                      Weight                                                ______________________________________                                        Quaternary Ammonium Silicate Solution,                                                                  200                                                 Quram 220 (PQ Corporation)                                                    Soluble Sodium Silicate Solution, PQ Corporation                                                        45                                                  Solution N                                                                    Ethylene-Vinylacetate Copolymer Dispersion,                                                             53                                                  Latiseal A7922, (Pierce and Stevens Company)                                  Ethylene-Acrylic Acid Solution Adcote 37-F-1,                                                           95                                                  (Morton Chemical Company)                                                     Monoethanolamine, Technical Grade                                                                       5                                                   Tamol 731-25, (Rohm & Haas Company)                                                                     0.25                                                Triton X-114, (Rohm & Haas Company)                                                                     0.25                                                Surfynol 104, (Air Products)                                                                            0.25                                                Lard Oil (Grade No. 1),   5                                                   (Mayco Oil and Chemical Co.)                                                  Mineral Oil (Gulf Security 155 or 71)                                                                   7.5                                                 Kaolin Clay, ASP 100 (Engelhart                                                                         100                                                 Mineral and Chemicals Company)                                                ______________________________________                                    

The ingredients recited in Table I are specifically described asfollows:

Quram 220: Methyl triethanol ammonium silicate solution, 45% SiO₂non-volatile content.

Soluble Sodium Silicate Solution N: 37.6% non-volatile solids, 3.22SiO₂/Na₂ O ratio.

Ethylene-Vinylacetate Copolymer Dispersion, Latiseal A7922 is adispersion of approximately 45% copolymer solids in water, a dispersionof a proprietary polymer estimated to comprise about 20-30% vinylacetateand the balance ethylene.

Ethylene-Acrylic Acid Solution, Adcote 37-F-1: 22% non-volatile solidsin ammonia, approximately 20% acrylic acid content in copolymer,designated as copolymer EAA 9300 by its manufacturer (Morton ChemicalCompany).

Tamol 731-25 (Rohm & Haas), Triton X-114 (Rohm & Haas), and Surfynol 104(Air Products) are all surfactants.

The formulated composition described in Table I is about 48%non-volatile solids and has a specific gravity of 1.30 and a weight pergallon of 10.8 pounds/gal.

                  TABLE II                                                        ______________________________________                                                          Approximate Time to Form                                    Chromate Reagent  a Rubbery or Hard Gel                                       ______________________________________                                        Chromic Acid, C.sub.r O.sub.3                                                                   5 minutes                                                   Ammonium Dichromate,                                                                            45 minutes                                                  (NH.sub.4).sub.2 Cr.sub.2 O.sub.7                                             Sodium Dichromate, Na.sub.2 Cr.sub.2 O.sub.7                                                    5 minutes                                                   Strontium Chromate, SrCrO.sub.4                                                                 greater than 60 days                                        Zinc Chromate, ZnCrO.sub.4                                                                      5 minutes                                                   Zinc Tetroxychromate,                                                                           5 minutes                                                   ZnCrO.sub.4.4Zn(OH).sub.2                                                     Potassium Chromate, K.sub.2 CrO.sub.4                                                           5 minutes                                                   Barium Chromate, BaCrO.sub.4                                                                    2 weeks*                                                    Lead Chromate, PbCrO.sub.4                                                                      greater than 60 days                                        Calcium Chromate, CaCrO.sub.4                                                                   40 minutes                                                  Calcium Dichromate, CaCr.sub.2 O.sub.7                                                          10 minutes                                                  Magnesium Chromate, MgCrO.sub.4                                                                 5 minutes                                                   ______________________________________                                         *Sample contains some traces of sodium and calcium salts as impurities        which accounts for the reduced stability observed.                       

On the basis of these observations, strontium chromate and lead chromateare the only two chromates which do not cause the silicate-resin vehicleto coagulate or gel. Because unmodified silicate-resin solutions(dispersions) tend to increase in viscosity with time, especially over aperiod of 60 to 90 days unless stored in cool surroundings, viscositybuild-up cannot be used to judge the effect of strontium or leadchromate on viscosity with time in storage. Increases in viscosity ofstrontium chromate modified silicate-resin mixtures over long periods oftime can be reversed by dilution of the mixture with water orwater-ammonia solutions and therefore the coatability of such mixturesremains fully viable. The irreversible coagulates or gels produced bythe other chromate reagents cannot be restored to coatability.

Concentration ranges of strontium chromate up to five percent of thesilicate-resin composition have been prepared successfully. Higherconcentrations provide little discernible improvement in corrosionresistance and impose a substantial economic burden on the composition.Premilling of the SrCrO₄ into a paste prior to adding it to thesilicate-resin coating system makes it easier to disperse this agentinto the remainder of the system to give a coating with good smoothnessand optimum levelling characteristics.

Duplicate samples, 4×6 inches, of electrical sheet coated with asilicate-resin coating modified with 3 percent by weight (on totalformula) of strontium chromate pigment were placed as cover plates over3.5-inch diameter petri-dishes filled to within 1/4-inch of their rimswith water. The samples/petri-dishes were stored at ambient roomtemperature so that the sheet surfaces above the water would alternatelycondense or evaporate water as the room temperature fluctuated. Thesample sheets were examined once every 24 hours for corrosion occurrenceor advancement. In this simple test a standard silicate-resin coating iscompletely rust stained within 24 hours of exposure (one day). Strontiumchromate modified coatings of 0.1 mil and 0.2 mil thickness (dry) showedless than 5 percent rust stain and less than 2 percent rust stain,respectively, over the areas exposed to the ambient humidity provided bythis test after 21 days of exposure. The percentage estimates are visualapproximation as compared to a completely stained unmodifiedsilicate-resin coated specimen. The rusting observed in the strontiumchromate-modified coating films were primarily of the pinpoint varietyearly in the exposure and these in some instances expanded to areas ofup to 1/4-inch diameter by the end of the 21-day exposure period. Inmost cases, however, the original pinpoint rust spots remained 1/16-inchor less in diameter. Overall, the test results clearly show the value ofthe strontium chromate modification of silicate-resin as a means ofprolonging the corrosion resistance of the system.

Dry coatings of my composition should be at least 0.05 mil thick.

I claim:
 1. Composition useful for coating substrates comprising asolution or dispersion in an aqueous medium ofa. quaternary ammoniumsilicate of the formula WM₂ O.X(N_(n) R_(p))₂ O.Y SiO₂ Z H₂ O where W isa number from 0 to 1, X is a number from 0.33 to 1.5, Y is a number from2 to 10, Z is a number from 1 to 20, M is an alkali metal, R is anorganic radical that forms an NR base selected from alkyl, alkanol, andheterocyclic groups, each independently selected, having from 1 to 20carbon atoms, n is an integer from 1 to 10, and p is at least 4, and b.ethylene polymer selected from the group consisting of copolymers,terpolymers and ionomers of ethylene with acrylic, methacrylic orcrotonic acids or esters thereof or vinyl acetate, wherein the portionof ethylene polymer derived from ethylene is from about 60 to about 90weight percent, the weight ratio of SiO₂ derived from the quaternaryammonium silicate to ethylene polymer being from about 20:1 to about1:20, and c. about 0.1% to about 5%, based on the total of a and b, of achromate selected from the group consisting of barium chromate,strontium chromate, and lead chromate.
 2. Composition of claim 1 whereinthe quaternary ammonium silicate solution is a solution of methyltriethanol ammonium silicate.
 3. Composition of claim 1 wherein theethylene polymer contains about 70 to about 85% ethylene.
 4. Compositionof claim 1 wherein the ratio of SiO₂ derived from the quaternaryammonium silicate to ethylene polymer is from about 14:20 to about 6:20.